Saponins are glycosidic compounds that are produced as secondary metabolites of steroids and triterpenes. They are widely distributed among plant species and in some marine invertebrates. The chemical structure of saponins imparts a wide range of pharmacological and biological activities, including some potent and efficacious immunological activity. Semi-purified saponin extracts from the bark of the South American Quillaja saponaria Molina tree (Quillaja saponins) exhibit remarkable immunoadjuvant activity. Because the Quillaja saponins are found as a mixture of at least one hundred structurally related saponin glycosides, their separation and isolation is often difficult if not prohibitive.
The most active fraction of these extracts, designated QS-21, has been found to include a mixture of two principal isomeric triterpene glycoside saponins, each incorporating a quillaic acid triterpene core, flanked on either side by complex oligosaccharides and a stereochemically rich glycosylated fatty acyl chain. The potency of QS-21 and its favorable toxicity profile in dozens of recent and ongoing vaccine clinical trials (melanoma, breast cancer, small cell lung cancer, prostate cancer, HIV-1, malaria) have established it as a promising new adjuvant for immune response potentiation and dose-sparing. However, the tolerated dose of QS-21 does not exceed 100 μg, above which significant local and systemic side effects arise.
Access to other potent Quillaja saponins has been hindered by difficulties in obtaining pure species from Quillaja saponin extracts. Furthermore, the structural identity of many Quillaja saponins remains only postulated. The discovery of new Quillaja saponins and related analogs with potent adjuvant activity and low toxicity presents a challenge to the fields of chemical synthesis and medicine.